1. Field of the Invention
The invention relates to a method and apparatus for the dynamic concentration of a suspension by means of a closed filter, and more particularly to such a method and apparatus in which a dynamic multi-stage pressure filter is equipped with a stirring device and the filter cake being formed is continuously remixed by stirring with the suspension to be dehydrated so that the same is kept in the liquid state until the desired degree of concentration is attained.
2. Description of the Prior Art
Techniques for dynamic concentration of a suspension including both method and apparatus, are well known in the art. For example, an account of such a technique is provided in a technical article in the periodical "Chemie-Ing.-Techn. 43, Vol. 1971/No. 6" at Pages 380-386.
Such dynamic filters are based on the operating principle of removing the solid coating, the so-called filter cake, which forms on the filter medium during the concentration of the suspension at the very moment that it arises. This is meant to prevent a build-up of a progressive resistance for the further feed of the suspension. In order to achieve this, therefore, the developing filter cake is continuously remixed during the concentrating process with the suspension and is repeatedly advanced to the filter medium and is thereby maintained in the liquid state by means of an energy feed until a viscosity is reached at which, given the influence of the aggitation of the stirring device, flowability of the filter cake and, thus, its dischargeability on the basis of the drop of pressure in the suspension space is just still retained.
In this method, there is a very small working latitude between the attainment of a satisfactory degree of concentration of the concentrate and the blocking of the filter due to the loss of flowability of the concentrate. This leads to the disadvantage of control problems which are extremely difficult because they are delicate.
According to the cited publication, it has therefore already been attempted to initiate an improvement of the operating conditions by means of intermittent opening and closing of the discharge for the concentrate. However, such a method is connected, in practice, with significant difficulties in terms of control engineering and, in addition, has the aggravating disadvantage of an excessive stress of the filter coatings. For continuous dynamic concentration, it has already been proposed in Austrian Pat. No. 290,563 to control the discharge of the thickened suspension as a function of the drive torque of the stirring device, whereby, in the case of the increase of the torque over a predetermined value, the element for discharging the thickened suspension is opened to a greater degree and, in the reverse case, it is closed to a greater degree or completely.
The known method, however, has the disadvantage that, on the one hand, the viscosity and/or moisture of the discharge concentrate fluctuates within broad limits. In addition, the filter media are subjected to a high stress due to changing pressure conditions or pressure surges. Also, given substances which tend toward spontaneous thixotropy, a blockage of the filter cannot be prevented with certainty. Furthermore, there is the disadvantage that the output of the filter is reduced and that, finally, in concentrating viscous media, particularly in multi-stage filters, the control operation requires an excessive amount of time until the same becomes effective so that, despite the control, disruptions due to the blockage of the filter and, therefore, shutdown of the operation can occur.